The subject invention is directed to antenna arrays and more particularly to a method and means for phasing the elements of active retrodirective antenna arrays.
An active retrodirective array (ARA) is an antenna array which automatically steers its transmitted beam towards the apparent source of an incoming pilot signal. The modifier "active" means that the radiated power is generated by sources associated with the antenna, rather than by reflection of an incident signal as in a passive retrodirective antenna (e.g., corner reflector).
Such arrays, which are also known as "self-focusing" arrays, have been suggested for some time. In such arrays, the transmitted wavefront duplicates the incoming pilot signal wavefront whatever its shape. The self-focusing property is important because it means that the transmitted power is focused back on the pilot source whatever the state of the intervening propagation medium, provided that the state persists for the round trip light time. Though not yet in practical use, ARA's are expected to become an important part of phased array technology. They have, for example, been proposed for microwave power-transmission from orbiting solar power stations, communication satellite transmitting arrays, and aircraft transponders.
The retrodirective properties of proposed ARA's is achieved by "conjugating" a pilot signal incident at each array element E.sub.i. At time t, an array element E.sub.i receives a pilot signal phase of the form EQU .phi..sub.pi =.omega.t-.beta.r.sub.i
where r.sub.i is the distance from the pilot source to the i.sup.th array element and .beta.=.omega./v where v=the phase velocity in the medium between the array and pilot source. To provide for retrodirectivity, the i.sup.th element in turn must transmit a signal which is the phase conjugate of the received signal of the form EQU .phi..sub.ti =.omega.'t+.beta.'r.sub.i +.phi..sub.0
where .phi..sub.0 is an arbitrary phase offset and .beta.'=.omega.'/v. To maintain precise retrodirectivity it is necessary that the frequencies .omega.' of the transmitted signal .phi..sub.ti and .omega. of the pilot signal .phi..sub.pi be coherent and that the phase offset .phi..sub.0 be identical for each of the array elements E.sub.i.
Perhaps the best known phase conjugation technique is the heterodyne type proposed by Skolnik et al at pp. 142-149, IEEE Transactions on Antennas and Propagation Vol. AP-12, March 1964. The simplest of such circuits merely generates 2.omega.t+.phi..sub.0 and substracts .phi..sub.pi in a mixer. Unfortunately, this simple technique cannot be realized with existing mixers due to their imperfect isolation. "Nearly" phase conjugating circuits, where the reference is slightly offset from 2.omega. have been built.
Another type of phase conjugation circuit uses a phase locked loop. Like the simplest heterodyne circuit, this circuit is impractical since it requires near perfect mixer balance. There are many ways around this problem, but all lead either to more complicated circuits or, as in the case of the simplest heterodyne circuit, to imperfect conjugation.
A third kind of phase conjugation circuit uses servoed phase shifters to bring the received phase into agreement with a phase reference. The transmitted signal passes through the same phase shifter and phase conjugation results from reciprocity. This technique is disclosed by Margerum at pp. 341-407 of Microwave Scanning Antennas, Vol. 3, Array Systems, Academic Press, N.Y., 1966.
Margerum's example of this circuit also employs "central phasing." This means that all the phase conjugation circuits are located in an electrically compact "central phasing unit" rather than at their respective array elements. Each phase conjugation circuit is connected to its array element by a bilateral transmission line. This connection avoids the problem of distributing a uniform phase reference to each of the many phase conjugating circuits of a large array.
One difficulty with the simple radially structured central phasing approach described by Margerum is that the central phasing unit of a very large array of, say 10,000 elements, would be so large that phase reference distribution within the unit would be a difficult problem. More importantly, the problem of switching over to a back-up reference element and its associated central phasing unit, should the main one fail, has no simple solution in a radially structured system.
However, the central phasing technique is of interest because it points out the possibility of achieving an ARA array wherein the retrodirective property is independent of how the elements are arranged or aligned in the array. Also, the retrodirective property is not affected by the motion of the antenna elements relative to one another or of the pilot source. The pattern (gain, sidelobes, etc.) of the ARA is of course, determined by these geometrical factors just as it is for any array, but the retrodirective property is not. While the use of phase shifters as conjugating elements is impractical due to their weight and relatively high RF losses and central phasing itself has several disadvantages, independence of retrodirectivity from geometrical factors is a highly desirable objective.
If this objective can be achieved in a practical system, it should be possible to fabricate rather light and floppy arrays because such arrays would only have to be stiff enough to maintain the shape (i.e., gain, sidelobe levels, etc.) of the pattern within specified limits. The direction of the beam would not be affected by deformations of the array structure. However, this structural flexibility can be achieved only if the phase stability of the phase reference distribution system of the antenna array can be made independent of its dimensional stability. Otherwise, phase errors due to structural deformation will induce pointing errors in addition to pattern distortions.
The pointing error problem can be especially acute for very large arrays in space, such as the envisioned 1.0 km diameter array proposed for a synchronously orbiting solar power satellite. This antenna will be required to transmit S-Band power to a ground antenna array less than 10 km in diameter. The required pointing accuracy will be of the order of 200 m (or about one second of arc at synchronous altitude), which would, if conventional phasing techniques were used, require that the transmitting array dimensions be constant within about 6 parts per million. Thermal expansion cycling due to the daily rotation of the array would surely exceed this limit, even if the array were built of materials exhibiting the very lowest thermal expansion possible.
From the above discussion, the need for more effective phase conjugating techniques to facilitate ARA implementation is clear. None of the ARA schemes described in the prior art have provided a sufficiently accurate and practical method for generation or distribution of the necessary phase reference to a plurality of antenna array elements. Particularly, a means of providing a constant reference offset .phi..sub.0 to a plurality of phase conjugation circuits, each of which is associated with a particular array element, has not been provided. Neither has this problem been particularly solved for the case of very large and distant antenna arrays described above.